1. Field of the Invention
The invention relates to a memory element, and more particularly to a phase-change memory element.
2. Description of the Related Art
Most electronic equipment uses different types of memories, such as DRAM, SRAM and flash memory or a combination of these memories based on the requirements of the application, the operating speed, the memory size and the cost considerations of the equipment. The current developments in the memory technology field include FeRAM, MRAM and phase-change memory. Among these alternative memories, phase-change memory will be the most likely to be mass manufactured in the near future.
Phase-change memory is targeted for applications currently utilizing flash non-volatile memory. Such applications are typically mobile devices which require low power consumption, and hence, minimal programming currents. A phase-change memory cell should be designed with several goals in mind: low programming current, higher reliability (including electromigiation risk), smaller cell size, and faster phase transformation speed. These requirements often set contradictory requirements on feature size, but a careful choice and alignment of materials used for the components can often widen the tolerance.
In order to reduce the programming current, the most straightforward way is to shrink the heating area. A benefit of this strategy is simultaneous reduction of cell size. Assuming a fixed required current density, the current will shrink in proportion to the area. In reality, however, cooling becomes significant for smaller structures, and loss to surroundings becomes more important due to increasing surface/volume ratio. As a result, the required current density must increase as heating area shrinks. This poses an electromigration concern for reliability. Hence, it is important to use materials in the cell which do not pose an electromigration concern. It is also important to improve the heating efficiency, by increasing heating flux in the active programming region while reducing heat loss to the surroundings.
The requirements above are best served by sandwiching the heating region between two regions of phase-change material, preferably the chalcogenide Ge2Sb2Te5 (GST). The thermal conductivity of this material is notably low, ˜0.2-0.3 W/m-K, due to the 20% presence of vacancies in the crystalline (fcc phase) microstructure. Heating is confined to a small area between a bottom and top portion of the chalcogenide material. A key aspect of this invention is the method of forming such a small area. The bottom portion is contained within a trench formed over the drain in one dimension, and the drain width in the other dimension. The top portion is an extended chalcogenide line perpendicularly oriented with respect to the trench formed over the drain. Preferably, this line is parallel to, of equal width to, and directly under the metal bit-line used to access the memory cell.
U.S. Pat. No. 5,789,758 assigned to Micron (“Chalcogenide Memory Cell with a Plurality of Chalcogenide Electrodes”) utilizes a pore in a dielectric layer positioned between an upper and lower chalcogenide electrode, both of which have greater cross-sectional areas than the pore. Formation of the pore in a dielectric layer is a very difficult task to do, and filling it with chalcogenide is even harder. Alternatively, formation of a chalcogenide island to be covered with dielectric is also difficult. Generally, three lithographic steps are needed to form this chalcogenide structure. It is desirable to minimize the number of lithographic steps to manufacture the device.
U.S. Pat. No. 7,034,332 assigned to HP (“Nanometer-Scale Memory Device Utilizing Self-Aligned Rectifying Elements and Method of Making”) utilizes rectifying elements disposed between a set of first electrodes and a set of second electrodes. In this case, it is difficult to form the rectifying element due to lithographic difficulty.
In order that fabrication process not be complicated, a phase-change memory element with a minimal number of process steps is called for.